Brian Eno has an extra feature to his ambient music. Occasionally there will be a little note out of place, a tiny surprise. He says this is to keep you from falling asleep, or rouse you out of a daydream you may have fallen into. If you're really concentrating, this little "bump" in the music probably won't even register.

This is featured in "Music for Airports" (good for busy places), and "Discreet Music" (takes some getting used to, but fades solidly into subconsciousness after a few minutes).